In the course of producing oil and gas wells, typically after the well is drilled, the well may be completed. One way to complete a well is to divide the well into several zones and then treat each zone individually.
Treating each section of the well individually may be accomplished in several ways. One way is to assemble a tubular assembly on the surface where the tubular assembly has a series of spaced apart sliding sleeves. Sliding sleeves are typically spaced so that at least one sliding sleeve will be adjacent to each zone. In some instances annular packers may also be spaced apart along the tubular assembly in order to divide the wellbore into the desired number of zones. In other instances when annular packers are not used to divide the wellbore into the desired number of zones the tubular assembly may be cemented in place.
The tubular assembly is then run into the wellbore typically with the sliding sleeves in the closed position. Once the tubular assembly is in place in the well and has been cemented in place or the packers have been actuated the wellbore may be treated.
The wellbore treatment typically consists of high pressure pumping of a viscous fluid containing proppants down through the tubular assembly out of the specified sliding sleeve and into the formation. The high-pressure fluid forms fractures, cracks and fissures in the formation and fills them with proppants. When the treatment ends, the proppants remain in the fractures, holding the cracks and fissures open and allowing wellbore fluid to flow from the formation zone, through the open sliding sleeve, into the tubular assembly, and then to the surface.
To open a sliding sleeve, an obturator, such as a ball, a dart, etc., is dropped into the wellbore from the surface and pumped through the tubular assembly. The obturator is pumped through the tubular assembly to the sliding sleeve where it lands on the seat of the sliding sleeve and forms a seal with the seat on the sliding sleeve to block all further fluid flow past the ball and the seat. As additional fluid is pumped into the well the differential pressure formed across the seat and ball provides sufficient force to move the sliding sleeve from its closed position to its open position. Fluid may then be pumped out of the tubular assembly and into the formation so that the formation may be treated.
In order to selectively open a particular sliding sleeve the obturator may be sized so that it will pass through the sliding sleeves until finally reaching the sliding sleeve where the seat size matches the size of the obturator. In practice the sliding sleeve with the smallest diameter seat is located closest to the bottom of toe of the well. Each sliding sleeve above the lowest sliding sleeve has a seat with a diameter that is slightly larger than the seat below it. By using seats that step up in size as they get closer to the surface, a small diameter obturator may be dropped into the tubular assembly and will pass through each of the larger diameter seats on each sliding sleeve above the lowest sliding sleeve. The obturator finally reaches the sliding sleeve with a seat diameter that matches the diameter of the obturator. The obturator and seat block the fluid flow past the sliding sleeve actuating the particular sliding sleeve.
Progressively larger obturators are launched into the tubular assembly to selectively open each sliding sleeve. Each seat and obturator must be sized so that the seat provides sufficient support for the obturator at the anticipated pressure. Currently there seems to be an upper limit on the number of sliding sleeves that may be actuated by progressively larger obturators and seats thereby limiting the productivity of a single well. An additional limitation of the current technology is that by utilizing progressively smaller seats towards the bottom of the well the productivity of the well is further limited as each seat chokes fluid flow from the bottom of the well towards the top of the well. Therefore in practice there is usually the additional step of drilling out the seats adding further costs to completing the well.